U.S. Pat. No. 3,842,194 of Clemens discloses a video disc having a playback system utilizing variable capacitance. In one configuration of the Clemens system, information representative of recorded picture and sound is encoded in the form of a relief pattern in a relatively fine spiral groove on the surface of a disc record. For example, groove widths about 3.5 micrometers and groove depths of about 1.0 micrometer may be used. During playback, a pickup stylus about 2.0 micrometers wide having a thin conductive electrode thereon, for example, about 0.2 micrometer thick, engages the groove as the record is rotated by a supportive turntable. Capacitive variations between the stylus electrode and the record surface are sensed to recover the prerecorded information.
Keizer, in U.S. Pat. No. 4,162,510 entitled, "Keel Tipped Stylus for Video Disc Systems", which is incorporated herein by reference, discloses a playback stylus which may be used with the Clemens system. The keep tipped pickup stylus comprises a dielectric support element having a body, a constricted terminal portion and shoulders interconnecting the body with the constricted terminal portion.
The constricted terminal portion is defined by a prow, a substantially flat rear surface remote from said prow, side surfaces extending from the side edges of the rear surface, a bottom surface, which preferably conforms to the groove shape, extending from the bottom edge of the rear surface and a plurality of additional surfaces extending from the prow and intersecting the bottom and the side surfaces. The maximum separation between the substantially parallel side surfaces is less than the given groove width.
A companion U.S. Pat. No. 4,104,832, incorporated herein by reference, discloses a method for manufacturing the keel tipped stylus. A tapered dielectric support element made from a hard material, such as diamond or sapphire, is contacted with an abrasive lapping disc having a deep, trapezoidal, coarse pitched groove. The lands on the lapping disc lap the shoulders of the keel tipped stylus and the walls of the abrasive groove form the substantially parallel side surfaces of the constricted terminal portion.
Abrasive discs can be generated by a method described by Ryan, "Material and Process Development for Video Disc Replication", RCA Review, Vol. 39, pages 87-115 (1978). A master recording is generally made in photoresist by optical recording techniques, although other recording methods such as electron beam or electromechanical means can also be employed. A metal matrix master, which is a negative replica of the surface of the photoresist master recording, is prepared from the master recording. A metal matrix mold, which is a positive replica of the master recording, is prepared from the matrix master. A metal stamper is then prepared from the matrix mold. The stamper is a negative of the photoresist master recording and is used to press plastic discs having the appropriate groove geometry which are then coated with an abrasive.
A problem has been encountered in using the above procedure. The keel-tipped styli prepared by the Keizer method sometimes break during playback, causing damage to the video disc and requiring replacement.